Current electroless metal baths cannot deposit metal on silicon or silicon-based compounds because silicon, as a semiconductor, has no catalytic activity for metal deposition. As an example, in order to deposit copper on silicon, the silicon surface is first immersed in a sensitizing bath, then metallized with a thin layer of palladium or other non-precious metal in a separate activation bath, followed by depositing copper over the palladium or other metal in an electroless plating bath. This method of metallization is both costly and involves many steps of processing.
The alternative method is sputter deposition of the metal on the silicon. This method requires expensive and complex process control to provide a uniform thickness of metal on the silicon.
Metal silicides are important in the electronics industry for chip fabrication. Polysilicon is used in chip fabrication to form self-aligned gates in CMOS technology and self-aligned emitters in bipolar technology. However, even when heavily doped, polysilicon is a poor conductor due to its high resistivity. The high resistivity of polysilicon makes it unsuitable for local interconnects. Metal silicides circumvent this problem by allowing the deposition of certain metals on polysilicon. The polysilicon can be used for the gate or emitter and then converted into a metal silicide which has a much lower resistivity.
A copper nitrate (Cu(NO.sub.3).sub.2) and ammonium fluoride (NH.sub.4 F) solution has been proposed as a means to deposit a thin layer of copper on silicon in a pair of articles by Zwicker and Kurtz, (Walter K. Zwicker and Stewart K. Kurtz, "The growth of silver and copper single crystals on silicon and the selective removal of silicon by electrochemical displacement," Acta Electronica, Vol. 16, No. 4, pp 331-338 (1973) and Walter K. Zwicker and Stewart K. Kurtz, "Anisotropic Etching of Silicon using Electrochemical Displacement Reactions," Second International Symposium on Silicon Material Science and Technology, Symposium Volume of the Electrochemical Society, Spring Meeting--Chicago, pp. 315-326, 1973.)
However, as the articles readily admit, the copper nitrate-ammonium fluoride solution causes flaking or blistering of the deposited copper on the silicon indicating a lack of adhesion between the copper and the silicon. Also, the copper can never be completely deposited over any meaningfully large area of silicon. And the deposits themselves tend to be non-uniform with no preferred orientation and ill-defined habits. Furthermore, this particular solution precipitates copper fluoride (CuF.sub.2).
A cupric chloride (CuCl.sub.2) and stannous fluoroborate (Sn(BF.sub.4).sub.2) colloidal catalytic composition is used as an intermediate step in an electroless metal deposition process in U.S. Pat. No. 4,634,619. However, this particular method requires an initial essential step of immersion in a polyelectrolyte pre-wetting bath, which substitutes for the traditional sensitizing bath, to allow the metal plating of a dielectric substrate.
Similar problems arise when attempting to fabricate patterned metal, such as metal conductor lines, on silicon or silicon compound substrates.
It is an object of this invention to provide a means of depositing metal on silicon.
It is another object of this invention to provide a means of fabricating patterned metal on silicon.
It is another object of this invention to provide a means of fabricating metal silicides on silicon.